In a mold-clamping device such as an injection molding machine and a press molding machine, a plurality of hydraulic pumps having a relatively small capacity are used and the number of the hydraulic pumps in operation is changed to correspond to a flow rate change according to a piston speed of a hydraulic cylinder so that the device can be compact in size and mold opening and closing can be accelerated.
In a case where the plurality of hydraulic pumps are used as described above, the plurality of hydraulic pumps are turned OFF one after another for a phased oil amount decrease when a piston of the hydraulic cylinder is decelerated from a high speed to a low speed. In this case, the amount of a hydraulic oil in the hydraulic cylinder is likely to be temporarily short of a required oil amount after initiation of the deceleration of the piston of the hydraulic cylinder so that a cylinder chamber on a supply side of the hydraulic cylinder has a negative pressure therein, a gap is generated in the cylinder chamber, and a temporary stop and a stick-slip occur in a movable mold plate.
According to a known technique, a main hydraulic pump in which the oil amount is controlled by a proportional solenoid valve is disposed so as to prevent the temporary stop and the stick-slip in the movable mold plate, and the main hydraulic pump is lastly and gradually decreased from a maximum oil amount when the plurality of hydraulic pumps are turned OFF one after another for the phased oil amount decrease with the main hydraulic pump at the maximum oil amount so that the hydraulic oil is prevented from having the negative pressure (for example, refer to PTL 1).
According to another known technique, a check valve is put into operation by using a negative pressure, a hydraulic oil is supplied to a hydraulic cylinder from a hydraulic oil tank, and the negative pressure is released in a case where the hydraulic cylinder has the negative pressure therein and a hydraulic circuit has the negative pressure so as to shorten the length of a stop time attributable to a temporary stop and a stick-slip in a movable mold plate (for example, refer to PTL 2).